11 Fertility characterization of slick hair Holstein cattle

In Puerto Rico, dairy cattle are exposed to moderate and severe levels of heat stress throughout the day and year-round. Slick-haired Holstein (SL) cattle, which are carriers of the slick mutation (InDel mutation in the PRLR gene (p.L462*)) have greater heat tolerance and have demonstrated lower vaginal and rectal temperature compared to the wild-type (WT, nonslick). We have hypothesised that SL could have reproductive advantages over the WT. The objective of this study was to compare the development of ovarian structures during the week of oestrous synchronization in Holstein heifers. In October 2022 (average temperature-humidity index (THI) = 76, maximum THI = 90), at the Agricultural Experiment Station in Gurabo, oestrus rates and ovarian structures development were recorded in SL (n = 5) and WT (n = 5) Holstein heifers. Oestrus was synchronized (5-day CoSynch) and ovarian structures were observed using a transrectal ultrasound system. Rectal temperature, follicle number, follicle size (i.e. small, medium, dominant) frequency, number, and size of corpus luteum and oestrus rate were evaluated. Results indicate that SL heifers maintain lower rectal temperature (38.45°C SL vs 38.65°C WT; P < 0.0001) compared to WT. At Day −1 relative to ovulation (Day 0), SL heifers have greater number of follicles in total (6.92 SL vs 5.42 WT; P < 0.0001). At Days −2, −1, and 0, WT heifers have greater frequency of medium follicles (WT vs SL = Day −2, 29.17% vs 13.04%; Day −1, 13.79% vs 8.11%; Day 0, 22.58% vs 16.13%) while the SL have greater frequency of dominant follicles (WT vs SL = Day −2, 20.83% vs 30.43%, Day −1, 6.90% vs 8.11%, Day 0 3.23% vs 9.68%). At Day 0 both groups present 40% of heifers in heat (2/5). In conclusion, there are detectable differences in ovarian structure development between genotypes that could be associated with improved fertility of the SL. Further, it is possible that the reproductive advantages that the SL show over the WT are due to changes in hormonal patterns that were not the objective of this study. Another possible explanation is that the uterine environment is more fertile in the SL Holstein compared to WT in tropical climates. Future studies are required to identify why the SL cows have shorter calving interval compared to WT.

This research was funded by USDA NIFA Hatch, Project 1025357.